How to Pass Extra Information from a Route to a Controller

Parameters inside the defaults collection don't necessarily have to match
a placeholder in the route path. In fact, you can use the defaults
array to specify extra parameters that will then be accessible as arguments
to your controller, and as attributes of the Request object:

As you can see, the $title variable was never defined inside the route
path, but you can still access its value from inside your controller, through
the method's argument, or from the Request object's attributes bag.